The present invention is directed to a method for removal of selected products from a mixed liquid hydrocarbon stream, particularly, the removal of propane from a natural gas liquids (NGL) stream.
A clean environment, particularly clean air, is quickly becoming a major issue on the world scene today. Research has been targeted to the reduction of automobile emissions, a primary source of air pollution. In this search for an alternate fuel source, propane has become recognized as a clean burning efficient fuel for motor transportation use. Many local governments are required to find a replacement for diesel power in busses and other municipal transportations systems. As the search continues for a cleaner burning fuel, a number of processes in the prior art relate to the recovery of propane from natural gas. These processes, however, are not particularly efficient, having a propane recovery rate of only about 65% to 70% of the propane content of the natural gas. In conventional methods, propane is recovered from natural gas which is cryogenically processed by refrigerating the gas to a low temperature to effect separation of the propane and heavier components from the natural gas stream. This is a relatively expensive process, requiring refrigeration equipment, piping, vessels, compressors and utilities to operate the plant.
In U.S. Pat. No. 4,251,249 to Jerry G. Gulsby issued on Feb. 7, 1981, a low temperature process for separating propane from a natural gas stream is disclosed. Gulsby discusses the conventional methods of recovering propane and the heavier fractions from natural gas, including cryogenically processed natural gas which requires a temperature of about -60.degree. F. to achieve a high efficiency of separation of the propane from the natural gas stream. To achieve the desired temperature range, a refrigerant such as Freon is utilized in the cryogenic process. Freon is very expensive and not environmentally desirable and therefore not practical for cooling large quantities of natural gas. Accordingly, conventional refrigeration techniques for recovering propane from natural gas have been too expensive to use, particularly for the relatively low recovery efficiency of propane.
In the process disclosed in U.S. Pat. No. 4,251,249 a low temperature fractioning process is utilized to separate condensible components from a hydrocarbon gas stream containing methane, ethane, propane and heavier hydrocarbon components. In that process, after dehydration, the inlet gas is successively passed through a series of refrigerations zones, for example heat exchangers, followed by a first separation stage to separate liquid condensate from the hydrocarbon gas. The gas stream is cooled further by expansion followed by a second separation stage to separate liquid containing C.sub.1, C.sub.2 and C.sub.3 hydrocarbons from the gas which contains C.sub.1 and C.sub.2 hydrocarbons. Both streams are fed to a de-ethanizer fractioning column, with the liquid entering at a point intermediate the ends and the gas being utilized in a reflux condenser internal to the column above the point of entry of the liquid stream. The C.sub.1, C.sub.2 hydrocarbon gas in a reflux condenser in the column provides the refrigeration required to condense the propane and heavier constituents of the inlet gas and reject the methane and ethane.
Also disclosed in U.S. Pat. No. 4,251,249 is a process for recovering propane from natural gas disclosed in U.S. Pat. No. 3,292,380 issued Dec. 20, 1966, wherein gas under pressure is pre-cooled and expanded through a turbine to produce a gas condensate mixture at a low temperature. Another process in the prior art utilizes a conventional turbo-expander plant to initially reject the methane content of the natural gas stream, thereby yielding a high ethane and heavier hydrocarbon component recovery from the stream. The ethane and heavier component recovery products are then fed to a de-ethanizer to reject a large portion of the ethane, leaving sustantially only the propane and heavier gasoline components. Although this process requires refrigeration equipment, the de-ethanization may be carried out at +60.degree. F. rather than -60.degree. F. This process however is very expensive requiring additional equipment to accomplish the two-stage process as well as increased maintenance cost to keep the turbo-expander plant operating.
Techniques known in the prior art for separating ethane, propane and heavier hyrdrocarbons include fractional distillation employing de-ethanizers, depropanizers, debutanizers, iso- and normal-butane splitters and depropanizers in a cascade configuration. In order to achieve the same degree of propane removal as described in the present invention, prior art methods would require either of the following two configurations: (1) a de-ethanizer tower whose bottom product, principally propane and heavier components, is fed to a depropanizer and reflux for the de-ethanizer tower generated by condensing all or part of the tower overhead vapor product with a suitable cooling medium, commonly refrigerant. The depropanizer would then separate the propane as the overhead product, and the butane and heavier components as the bottom product; or (2) a depropanizer tower whose overhead product, principally ethane and propane, is fed to a de-ethanizer, and reflux for the depropanizer tower would be generated by condensing all or part of the tower overhead with a suitable cooling medium, commonly air or water. The bottoms from the depropanizer would comprise butane and heavier components. The net overhead product from the depropanizer tower would be fed to the de-ethanizer tower which then separates the ethane as the overhead product and the propane as the bottom product. Reflux for the de-ethanizer tower would be generated by condensing all or part of the tower overhead vapor product with a suitable cooling medium, commonly refrigerant.
Other techniques and processes are known in the prior art for separating propane and heavier hydrocarbons from a natural gas stream. These processes however have not yet been widely used because of the substantial expense required to operate such facilities, particularly when the propane recovery efficiency is low where a high rejection of ethane is attained. Accordingly, it is an object of the present invention to provide a process for the separation of propane from a mixed liquid hydrocarbon stream at substantially ambient temperature and high propane recovery efficiency.